It is general that a nitride semiconductor layer is formed on an inexpensive silicon substrate or sapphire substrate. However, lattice constants of these substrates are greatly different from a lattice constant of the nitride semiconductor layer, and they also have different thermal expansion coefficients. Thus, considerable strain energy is produced in the nitride semiconductor layer formed on a substrate by epitaxial growth. As a result, cracks are likely to be produced or crystal quality is likely to be lowered in the nitride semiconductor layer.
To solve the problem, a buffer layer having a nitride semiconductor layer with a different composition stacked thereon is arranged between the substrate and a functional layer formed of a nitride semiconductor in conventional examples.
Further, in order to improve characteristics of the nitride semiconductor layer, various suggestions about a buffer layer have been made. For example, Patent Document 1 discloses that boron is contained in the buffer layer in order to reduce pit density (i.e., defect density).
On the other hand, there has been known that a longitudinal breakdown voltage is improved by doping the buffer layer with iron. For example, in a semiconductor base substance disclosed in Patent Document 2, a longitudinal breakdown voltage of a buffer layer is increased by doping iron.